Plant constituents have been proven useful in the prevention and treatment of a wide variety of diseases and conditions. For example, barley has been shown to be particularly effective in lowering lipid levels in animal models (A. A. Qureshi et al., "Suppression Of Cholesterogenesis By Plant Constituents", Lipids, 20, pp. 817-24 (1985)). More specifically, .alpha.-tocotrienol, a chromanol isolated from barley extract, has been identified as a therapeutic agent for hypercholesterolemia (A. A. Qureshi et al., "The Structure Of An Inhibitor Of Cholesterol Biosynthesis Isolated From Barley", J. Biol. Chem., 261, pp. 10544-50 (1986)). In addition, tocotrienol, .gamma.-tocotrienol and .delta.-tocotrienol have also been shown to reduce hypercholesterolemia in mammals (European patent application 421,419).
Hypercholesterolemia involves high serum cholesterol levels and is a causative agent of diseases including arteriosclerosis, atherosclerosis, cardiovascular disease and xanthomatosis. In addition, high serum cholesterol levels are seen in patients suffering from diseases including diabetes mellitus, familial hypercholesterolemia, acute intermittent prothyria, anorexia nervosa, nephrotic syndrome, primary cirrhosis and various liver disorders, such as hepatitis and obstructive jaundice. Improvement of lipoprotein profiles and a decrease in total serum and low density lipoprotein cholesterol have been shown to retard the progression of such diseases, as well as to induce regression of clinically significant lesions in hypercholesterolemic patients.
Although the relationship between hypercholesterolemia and its many associated diseases, most notably cardiovascular disease, has been extensively studied, no clear answer to this world-wide problem has yet been found. As a result, coronary artery disease remains the leading cause of death in the United States and other developed countries. Coronary artery disease is the result of complex interactions between a large number of different processes, including lipoprotein metabolism, aggregation of blood platelets, blood coagulation and fibrinolysis. Accordingly, the cardiovascular risk profile of a given patient is dependent on these interactions.
In addition to lowering cholesterol levels, the cardiovascular risk profile of a patient may also be reduced by decreasing the levels of other factors in the serum and the blood. For example, reduction of thromboxane A.sub.2 generation (measured by the levels of thromboxane B.sub.2, a stable metabolite of thromboxane A.sub.2) and platelet factor 4 levels in the serum lessens the risk of cardiovascular disease because of decreased thrombogenic activity.
Thromboxane A.sub.2 and platelet factor 4 levels are also associated with other biological activities. For example, when reduction of these factors is accompanied by a reduction in macrophage cell count, lower tumor necrosis factor (TNF) levels and lower arachidonic acid levels in bodily tissues, reduced levels of prostaglandins, leukotrienes and interleukins are implicated. Reduction of these factors, therefore, leads to a decrease in the inflammation accompanying a wide variety of diseases. In addition, since prostaglandins inhibit glucose-induced insulin release and increase glucagon secretion, an increased insulin to glucagon ratio may also result from the reduction in prostaglandins. Such an increase is useful in improving glucose intolerance in diabetes mellitus and restoration of acute glucose-induced insulin response in non-insulin-dependent diabetes mellitus.
It has been noted that there is a low incidence of cardiovascular disease in populations consuming large amounts of cereal grains. Soluble and insoluble fibers have, in the past, been viewed as the agents responsible for cholesterol reduction in such populations (see D. Kritchevsky et al., "Fiber, Hypercholesterolemia and Atherosclerosis", Lipids, 13, pp. 366-69 (1978)). Recently, the hypocholesterolemic effects of cereal grains have been attributed to natural components of the grains--tocotrienols ("T.sub.3 ") and structurally similar compounds, such as tocopherols ("T"). Tocotrienols and tocopherols occur naturally in small quantities in a wide variety of plant sources, such as rice bran, palm oil and barley (A. A. Qureshi et al., "Lowering of Serum Cholesterol in Hypercholesterolemic Humans by Tocotrienols (Palmvitee)"., Am. J. Clin. Nutr., 53, pp. 1021S-6S (1991)).
As a-class, the tocopherols, including d-.alpha.-tocopherol (vitamin E), have been extensively studied. As a result of these studies, certain biological activities have been attributed to the tocopherols. Such activities include platelet aggregation and antioxidant functions (see, for example, E. Niki et al., "Inhibition of Oxidation of Biomembranes By Tocopherol", Annals of the New York Academy of Sciences, 570, pp. 23-31 (1989) and K. Fukuzawa et al., "Increased Platelet-Activating Factor (PAF) Synthesis in Polymorphonuclear Leukocytes of Vitamin E-Deficient Rats", Annals of the New York Academy of Sciences, 570, pp. 449-453 (1989)). Although the exact structure-function relationship is not known, several experiments have highlighted the importance of the phytyl side chain in the biological activity of tocopherols (see W. A. Skinner et al., "Antioxidant Properties of .alpha.-Tocopherol Derivatives and Relationships of Antioxidant Activity to Biological Activity", Lipids, 5(2), pp. 184-186 (1969) and A. T. Diplock, "Relationship of Tocopherol Structure to Biological Activity, Tissue Uptake, and Prostaglandin Biosynthesis", Annals of the New York Academy of Sciences, 570, pp. 73-84 (1989)).
In contrast to the tocopherols, interest in the tocotrienols has been limited, as those compounds were not typically considered to be biologically useful. Recently, however, studies have indicated that tocotrienols may be biologically active. For example, U.S. Pat. No. 4,603,142 identifies d-.alpha.-tocotrienol, isolated from barley extracts, as an inhibitor of cholesterol biosynthesis. See also A. A. Qureshi et al. (1986), supra. Various human and animal studies have confirmed the impact of pure tocotrienols, isolated from barley, oats and palm oil, on cholesterol biosynthesis, specifically LDL-cholesterol (A. A. Qureshi et al., "Dietary Tocotrienols Reduce Concentrations of Plasma Cholesterol, Apolipoprotein B, Thromboxane B.sub.2 and Platelet Factor 4 In Pigs With Inherited Hyperlipidemias", Am. J. Clin. Nutr., pp. 1042S-46S (1991); A. A. Qureshi et al., "Lowering Of Serum Cholesterol In Hypercholesterolemic Humans By Tocotrienols (Palmvitee)", Am. J. Clin. Nutr., 53, pp. 1021S-26S (1991); D. T. S. Tan et al., "The Effect Of Palm Oil Vitamin E Concentrate On The Serum And Lipoprotein Lipids In Humans", Am. J. Clin. Nutr., 53, pp. 1027S-30S (1991)). In addition, tocotrienol, .gamma.- and .delta.-tocotrienol have been indicated for use in the treatment of hypercholesterolemia, hyperlipidemia and thromboembolic disorders (European patent application 421,419).
The five known naturally occurring tocotrienols have been designated tocotrienol, .alpha.-, .beta.-, .gamma.- and .delta.-tocotrienol. Those compounds exhibit varying degrees of hypercholesterolemic activity and have also been used as antithrombotic agents and antioxidants. .alpha.-T.sub.3, for example, displays antioxidant activity against lipid peroxidation in rat liver microsomal membranes and against oxidative damage of cytochrome P-450 (E. Serbinova, Free Radical Biology and Medicine, in press (1991)). Despite these activities, the known tocotrienols have not found wide-spread therapeutic use.
Accordingly, the need still exists for compounds which, as single agents, can safely and effectively act as hypercholesterolemic, antithrombotic, antioxidizing, antiatherogenic, antiinflammatory and immunoregulatory agents.